Single barrel syringes for sequential injection of two or more fluid medications are known. These syringes generally include one or more floating plugs or pistons which divide the interior of the syringe barrel into chambers for different fluid medications. Examples of such multiple dose, single barrel syringes are shown in U.S. Pat. Nos. 3,911,916 to Stevens, 3,923,058 to Weingarten, and 4,439,184 to Wheeler. Despite the obvious advantages (particularly patient comfort) in using a single syringe capable of delivering two or more medications sequentially, instead of using separate syringes for each medication, multiple dose syringes have not enjoyed wide-spread use. There are several reasons why multiple dose, single barrel syringes have not become commercially successful. First of all, the floating plug(s) may tilt. This may happen either when one draws back the plunger to aspirate a small amount of blood from the patient, or when one presses the plunger forward to expel fluid. In either case, tilting of the plug greatly increases friction and makes it very difficult to press the plunger forward in order to expel fluid. This tilting also makes it very difficult to aspirate, since tilting is very likely to occur during aspiration. The second problem is that the plug, in order to be fluid tight (which it must be), has a rather high friction resistance against the barrel wall even when it does not tilt. The third problem is that these plugs, which are generally of substantial thickness, offer considerable resistance to puncturing by the needle. Each plug must be punctured in turn by the needle as the chamber on the forward or distal side of the plug is empty, in order to place the next chamber in communication with the needle. Difficulty in puncturing the plug poses a considerable obstacle to successfull use of the syringe.
Syringes having two or more barrels and a single needle for injecting a plurality of fluid medications are also known. The multiple barrel arrangement avoids the difficulties associated with the floating plugs in single barrel, multiple dose syringes. However, the multiple barrel syringes, even when only two barrels are present, are relatively bulky and difficult to use. Such syringes likewise have not enjoyed any appreciable commercial use.